Drawn and ironed metal containers have been in existence for a number of years and are utilized for packaging a variety of products. In the packaging of some of these products, it is necessary to provide a coating on the interior surface of the metal container to prevent the packaged product from picking up or absorbing some of the metal from the container wall and to protect the container from corrosive products. Normally, this coating is applied by introducing a spray nozzle through the open end of the container so that the container is coated along the bottom end wall, as well as the side wall. Because of the particular configuration of the end wall or bottom wall, it is sometimes difficult to obtain adequate or complete coating coverage on the bottom wall of the container.
Thus, it has been customary to do random testing for coating integrity prior to introducing the product into the container. It is well known to utilize an electrolytic solution along with a fixed voltage source and a probe placed into the solution to determine coating coverage integrity in coated containers. It is also well known that the type of solution, probe materials, polarity of test, the time duration of the test, and the voltage values are factors that determine the current value and, therefore, the coated container integrity.
One prior art-type of testing device that has been developed and is marketed by Waco (Wilkins-Anderson Co.), Chicago, Illinois. The testing device consists of a test fixture which supports and makes electrical contact with the container that has the electrolytic solution contained therein. A probe is introduced into the electrolytic solution and a fixed voltage source is applied to the probe and the current is recorded after a predetermined time period to ascertain coating integrity. In the prior art device, the probe is maintained "negative", while the container is maintained "positive", so that the conventional current flow is from the container through the solution to the probe.
Usually, the fixed voltage is approximately 6 volts in the prior art-type of testing device. The voltage was fixed and the current was the variable. A low current reading means that the coating integrity of the container is acceptable, while a high current reading means that the container is unacceptable.